This invention relates to a pressure sensitive switch for detecting leakage of fluid based upon detection of pressure change in various fluids such as gas, liquid, and so on, and in particular relates to a pressure sensitive switch which has a wide range of applications to various fluids and allows very compact design.
Normally, a pressure sensitive switch of this type is so designed that pressure change of fluid is detected by way of a pressure receiving element which responds to such pressure change of fluid (see for instance Japanese Patent Laying Open Publication Serial No. 50-27078 and Japanese Patent Laying Open Publication Serial No. 56-120044).
However, a diaphragm made of thin rubber is used in such a pressure receiving element as described in the above patent applications, and because it does not have a sufficiently good anti corrosive property and is not properly and reliably air tight, the application of such a pressure switch is limited to areas where such heavy duty anti corrosion performance is not required Furthermore, if on the other hand, a diaphragm or a bellows made of metal is used, because of some structural complications when combining it with a contact mechanism, there is a difficulty in achieving a compact design for the switch.
It might be thought worth while to consider a method which reduces the thickness of a metallic bellows by etching, but according to such a method it is difficult to obtain uniformity in thickness, and, if the thickness is excessively reduced, pinholes may be created, and by reducing the thickness the pressure receiving portion becomes so soft that there will arise some fluctuations in the contact position and the pressure position in relation with the switching mechanism, thereby impairing its performance.
If an electrodeposited bellows which can be made extremely thin by the process of electroplating is used, fluctuations of the plate thickness of the bellows are reduced without forming any pinholes, and therefore it can be used as a highly sensitive pressure switch which can accurately detect very slight pressure changes.
However, conventionally, to the end of supporting the pressure receiving element, a support member which provides a mounting seat surface for the pressure receiving element is fixedly secured to the case by screws. Therefore, the support member has to be sufficiently thick for allowing the screws to be threaded thereinto, and therefore it has been difficult to make the design of such a switch compact enough. Furthermore, depending on the magnitude of the fastening force for the screws, and the fluctuations in the fastening force, the mounting precision may be impaired, and the operating properties of the switch may be adversely affected. Additionally, the need for female and male threads and the increased number of component parts and additional labor involved all have in the prior art contributed to some increase in cost.
Another problem that has arisen will now be outlined. As the pressure receiving element mentioned above, a bellows is widely used because it is superior in repeated stress performance and is highly durable, as per se known, and for mounting this bellows normally the open end of the bellows is for instance fitted into a fixed member.
Therefore, the open end of the bellows is provided for instance with a fitting portion consisting of a straight tubular portion extending in the axial direction to a substantial extent, and therefore the bellows tends to be long, thereby making it hard to design the pressure switch compact enough. Also, in mounting a bellows, it is difficult to position it in relation with the direction of receiving the pressure, and high dimensional precision is required between the bellows and the corresponding fixed member for proper fitting, thereby increasing its cost.
Another problem that has occurred with regard to the prior art is that the operation of such a bellows is typically without any hysteresis property. Therefore, the point at which the switch shifts from ON to OFF and the point at which the switch shifts from OFF to ON are always identical, and as a result there have been problems such as chattering, and other unstable actions may develop, and it is hard to distinguish between the time of abnormal pressure drop (caused by gas leakage and so on) and the time of restored state (the normal state).
Additionally, since conventionally the sheet spring making up the switch is made as a single sheet, the switching actuating force required therefor is as high as 400 to 600 grams, and it has been difficult to achieve a light switching action force.